Nottinghamshire Police said all four remain in custody and enquiries are ongoing.

Tiara 'still missing'

The force previously released CCTV footage showing a trio of raiders using power tools to break into the glass case where the tiara was housed.

It also appealed for information about a silver Audi RS5 Quattro, registration KY61 USJ, found burnt out in Cross Lane, Blidworth, about 30 minutes after the burglary.

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Media captionCCTV shows the moments before the Portland Tiara was stolen

Det Insp Gayle Hart said: "The stolen property hasn't been recovered and we still want to hear from anyone who knows where the items might be."

The Portland Collection at Welbeck Abbey - home to the Dukes of Portland since 1607 - is based in the estate's converted gas works, and is made up of paintings, jewellery and other art collected by the Dukes over 400 years.

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The Portland Collection is part of the Harley Gallery which is housed on the estate